The present invention relates to fluid flow controllers.
Fluid flow controllers may be used in systems which control flow of fluids (e.g., gases) by sealing and unsealing a valve in a fluid path. In these systems, flow valves modulate fluid flow by adjusting the gap of an orifice. When the fluid flow required is zero, a positive pressure, i.e., a preload, is applied to the seal, e.g., to ensure its integrity against surface imperfections and positive line pressure increases that tend to open the valve.
It is known to utilize integrators to control flow valves. These integrators steadily increase or decrease a control signal, which controls the output power applied to he control valve, based on a constant difference between a set point valve, which is based on desired flow, and an actual flow valve, which is based on the actual flow. Thus, flow through the valve equals the desired flow over time. Because the valve has a preload to ensure the seal, when a set point is commanded, the output power has to ramp from zero to an amount necessary to overcome the sealing preload before the valve begins to open.
It is known to alleviate this problem by providing the controller with a bias adjustment, thus enabling the operator to set the valve to the just closed point before inputting a set point value. It is also known to sense both the flow and the set point and if the flow is zero when the set point is non-zero, to artifically insert a large error signal into the integrator, thus causing the control signal outputted from the integrator to ramp up much faster. This false error signal is removed when the flow sensor detects the onset of fluid flow.